Developing device

Abstract

To make compatible both the improvement of the exfoliation performance of a developer and the size reduction of a device.SOLUTION: A distance between a first maximum peak position at which the magnetic flux density of a first magnetic pole is maximum in a normal direction of a developing sleeve, and a second maximum position at which the magnetic flux density of a second magnetic pole is maximum in the normal direction of the developing sleeve is shorter than 18 [mm], and a distance along an external peripheral face of the developing sleeve of a width of a value which is obtained by adding 2 [mT] to a value at which the magnetic flux density is minimum in the normal direction of the developing sleeve at a downstream side with respect to the first maximum peak position, and at an upstream side with respect to the second maximum peak position is equal to or longer than 5.5 [mm].SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a developing device that develops an electrostatic latent image formed on an image carrier. [Background technology]

[0002] The developing device includes a developing sleeve as a rotatable developer carrier that carries a two-component developer (hereinafter, simply referred to as developer) containing toner and carrier. A magnet that has a plurality of magnetic poles and generates a magnetic field for carrying the developer on the surface of the developing sleeve is fixedly disposed inside the developing sleeve in a non-rotating manner. The developer carried on the surface of the developing sleeve is transported to a region (developing region) where development is performed on an electrostatic latent image formed on a photoconductor drum as an image carrier. When the developer carried on the surface of the developing sleeve is used for development in the developing region, the toner in the developer is consumed, and the toner concentration in the developer decreases.

[0003] The developer, whose toner concentration has decreased as a result of being used in development, is transported to a region (separation region) where a repulsive magnetic field is formed between the magnetic poles of the same pole of the magnet. In this separation region, the developer, whose toner concentration has decreased as a result of being used in development in the development region, is caused to separate from the development sleeve. The developer that has been separated from the development sleeve in the separation region is then collected in a development container. The developer collected in the development container is mixed with the developer already present in the development container, thereby making the toner concentration uniform.

[0004] If the developer used for development is not sufficiently removed from the developing sleeve, the developer with a reduced toner concentration will be used for development again in the development area as the developing sleeve rotates. As a result, the toner concentration in the developer used for development will become uneven, and there is a risk of image defects occurring. This is particularly problematic when forming a solid image. Therefore, it is necessary to sufficiently remove the developer used for development with a reduced toner concentration from the developing sleeve.

[0005] In the developing device described in Patent Document 1, a rotatable scraping roller equipped with a magnet body inside is disposed near the scraping magnetic pole of the magnetic field generating means. In this developing device, the magnetic pole inside the scraping roller attracts the developer carried by the developer carrier to the scraping roller side. This improves the scraping performance of the developer near the scraping magnetic pole. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2002-148921 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to ensure the peeling property of the developer, it is necessary to secure a sufficient distance along the outer circumferential surface of the developing sleeve between the magnetic poles of the same polarity that form the peeling region. However, in recent years, the developing device has become smaller, and the outer diameter of the developing sleeve has become smaller accordingly. If the number of magnetic poles of the magnet is the same, the smaller the outer diameter of the developing sleeve, the shorter the distance along the outer circumferential surface of the developing sleeve between the magnetic poles tends to be. As a result, the distance along the outer circumferential surface of the developing sleeve between the magnetic poles of the same polarity that form the peeling region cannot be secured sufficiently, and the peeling region becomes narrow, so that the developer used for development becomes difficult to separate from the developing sleeve (the peeling property of the developer is reduced).

[0008] If a configuration is adopted in which an additional magnet is added to the outside of the developing sleeve near the peeling magnetic pole, as in the case of the peeling roller described in Patent Document 1, space is required within the developing device to place the additional member outside the developing sleeve, which results in an increase in the size of the device.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a developing device that is capable of improving the developer stripping performance while also being compact in size. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention One The developing device according to the embodiment has the following configuration: That is, the developing device includes a rotatable developing sleeve that carries and conveys a developer containing toner and a carrier to develop an electrostatic latent image formed on an image carrier, and a magnet that is fixedly arranged inside the developing sleeve and non-rotatably, and has a first magnetic pole and a second magnetic pole that is arranged adjacent to the first magnetic pole downstream of the first magnetic pole in the rotation direction of the developing sleeve and has the same polarity as the first magnetic pole, and the magnetic flux density of the second magnetic pole is changed from a first maximum peak position where the magnetic flux density of the first magnetic pole is maximum in the normal direction of the developing sleeve to a second maximum peak position where the magnetic flux density of the second magnetic pole is maximum in the normal direction of the developing sleeve. The distance along the outer peripheral surface of the developing sleeve to the second maximum peak position where the flux density is at its maximum value is shorter than 18 mm, and when the direction of the force of the magnetic attraction force in the normal direction of the developing sleeve that attracts the developer toward the center of the developing sleeve is negative, the distance along the outer peripheral surface of the developing sleeve of the repulsive force width at which the magnetic attraction force in the normal direction of the developing sleeve is positive, downstream of the first maximum peak position and upstream of the second maximum peak position in the rotational direction of the developing sleeve, is 4.7 mm or more. Effect of the Invention

[0012] According to the present invention, it is possible to improve the developer stripping performance while reducing the size of the device. [Brief description of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view showing a configuration of an image forming apparatus according to a first embodiment. [Diagram 2] 1 is a cross-sectional view showing a configuration of a developing device according to a first embodiment. [Diagram 3] FIG. 2 is a schematic diagram illustrating a configuration of a developing device according to the first embodiment. [Figure 4]FIG. 2 is a diagram showing the distribution of magnetic flux density Br in the configurations of Example 1, Example 2, and the comparative example. [Diagram 5] FIG. 13 is a diagram showing the distribution of magnetic attraction force Fr in the configurations of Example 1, Example 2, and the comparative example. [Figure 6] FIG. 1 is a diagram showing the distribution of ∂Br / ∂r in the configurations of Example 1 and Comparative Example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the present invention, and not all of the combinations of features described in the embodiments are necessarily essential to the solution of the present invention. The present invention can be implemented in various applications, such as printers, various printing machines, copiers, FAX machines, and multifunction machines.

[0015] [First embodiment] (Configuration of Image Forming Apparatus) First, the configuration of an image forming apparatus according to a first embodiment of the present invention will be described with reference to the cross-sectional view of FIG.

[0016] As shown in Figure 1, the image forming device is a so-called tandem type. Drum cartridges for forming toner images of four colors, yellow, magenta, cyan, and black, are installed side by side, but in the following explanation, the reference characters A, B, C, and D are omitted and only the numbers are shown to indicate parts common to the four color drum cartridges in Figure 1.

[0017] The image formation in this image forming apparatus is performed as follows. That is, the surface of the photosensitive drum 1 is uniformly charged by the charging device 2. The charged surface is exposed to the laser 3 to form an electrostatic latent image on the photosensitive drum 1. The electrostatic latent image thus obtained is developed into a toner image by attaching toner from the developing device 4. This toner image is transferred onto the intermediate belt 62 by the primary transfer roller 61. Then, after the toner images of multiple colors are transferred onto the intermediate transfer belt 62 in a superimposed manner, the four-color toner image is transferred onto the recording medium conveyed from the paper feed cassette to the secondary transfer section where the secondary transfer roller 63 and the secondary outer transfer roller 64 come into contact. The recording medium is heated and pressed by the fixing device 7 to be fixed, and then discharged outside the image forming apparatus. Residual toner remaining on the photosensitive drum 1 after the transfer is removed by the cleaner 8.

[0018] (Configuration of the developing device) Next, the configuration of the developing device 4 will be described with reference to the cross-sectional view of FIG. 2 and the schematic view of FIG.

[0019] The developing device 4 has a rotatable non-magnetic developing sleeve 41 (developing rotor) that carries and transports a developer containing toner and carrier to develop the electrostatic latent image formed on the photosensitive drum 1. Inside the developing sleeve 41, a magnet 42, which is a magnetic field generating means, is fixedly disposed so as not to rotate.

[0020] The developing device 4 also includes a regulating blade 43, which is a developer regulating member that regulates the height of the magnetic brush formed on the developing sleeve 41 (the amount of developer carried on the surface of the developing sleeve 41), and a developing container 44 that contains a developer including toner and a carrier. The developing container 44 is partitioned into a developing chamber 44a and a stirring chamber 44b by a partition wall 47 that extends vertically. A first screw 45a is disposed in the developing chamber 44a, and a second screw 45b is disposed in the stirring chamber 44b. The first screw 45a stirs the developer in the developing chamber 44a and transports the developer in a first direction. The second screw 45b stirs the toner supplied from the toner supply layer and the developer in the stirring chamber 44b, and transports the developer in a second direction opposite to the first direction to make the toner concentration uniform.

[0021] In the first embodiment, the developer contained in the developing container 44 is a two-component developer in which a negatively charged non-magnetic toner and a magnetic carrier are mixed. The non-magnetic toner is a resin such as polyester or styrene that contains colorants, wax components, etc., and is pulverized or polymerized to form a powder. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, and a resin coating is applied to the surface layer.

[0022] The developing sleeve 41 used in the first embodiment has a cylindrical shape with an outer diameter of 20 mm or less, and is made of aluminum. Specifically, in the first embodiment, the outer diameter of the developing sleeve 41 is 18 mm. In addition, regular recessed grooves are formed on the outer peripheral surface of the developing sleeve 41 in the longitudinal direction of the developing sleeve 41 (the direction of the rotation axis of the developing sleeve). When viewed in a cross section perpendicular to the rotation axis of the developing sleeve 41, the grooves formed on the outer peripheral surface of the developing sleeve 41 are triangular in shape with a depth of 60 μm and a width of 120 μm, and 60 grooves are carved at equal intervals in the circumferential direction.

[0023] That is, in the circumferential direction of the developing sleeve 41, a plurality of grooves are formed over the entire outer peripheral surface of the developing sleeve 41, and each groove is formed at a predetermined interval in the circumferential direction of the developing sleeve 41. The grooves are closely related to the developer transport force of the developing sleeve 41, and the transport force increases as the depth, width, and number of grooves are increased, making it possible to stabilize the amount of developer carried by the developing sleeve 41 over a long period of time. On the other hand, if the transport force is excessively increased by the grooves, it becomes difficult to peel the developer from the surface of the developing sleeve 41.

[0024] The detailed configuration of the magnet 42 will be described. The magnet 42 used in the first embodiment is composed of five mag pieces 42a to 42e. The magnet 42 used in the first embodiment has a plurality of magnetic poles, and the number of magnetic poles is five. The developing sleeve 41 rotates in the direction of the arrow (counterclockwise) shown in FIG. 2, and transports the developer attracted at the position of the drawing-up magnetic pole S1 pole created around the first mag piece 42a toward the regulating blade 43. The developer made to stand up by the regulating magnetic pole N1 pole created around the second mag piece 42b has its layer thickness regulated by the regulating blade 43, and when it passes through the gap between the developing sleeve 41 and the regulating blade 43, it forms a developer layer of a predetermined thickness on the developing sleeve 41. The developer layer is carried and transported to the developing area facing the photosensitive drum 1, and develops the electrostatic latent image formed on the surface of the photosensitive drum 1 in a state in which magnetic poles are formed by the developing magnetic pole S2 pole created around the third mag piece 42c. That is, the development area facing the photosensitive drum 1 is the area where the magnetic brush comes into contact with the surface of the photosensitive drum 1. After being used for development, the developer passes through the transport magnetic pole N2 formed around the fourth mag piece 42d and is transported to the peeling area.

[0025] 2, the pumping magnetic pole S1 is disposed adjacent to the peeling region S3 downstream of the peeling pole S3 in the rotational direction of the developing sleeve 41, and has the same polarity as the peeling pole S3. Therefore, the developer carried on the surface of the developing sleeve 41 is peeled off by a repulsive magnetic field formed between the magnetic poles of the same polarity of the magnet (i.e., between the peeling pole S3 and the pumping magnetic pole S1). Hereinafter, the region between the peeling pole S3 and the pumping pole S1 in the rotational direction of the developing sleeve 43 is referred to as the peeling region.

[0026] In the peeling region, the developer is peeled off by the repulsive magnetic field of the peeling magnetic pole S3 and the pumping magnetic pole S1, which are the same polarity. The peeled developer falls into the development chamber 44a, is stirred and transported by the first screw 45a, and is then attracted to the development sleeve 41 by the pumping magnetic pole S1. If the developer is not sufficiently peeled off in the peeling region, the developer that has been used for development once and has a reduced toner concentration is pumped up to the development sleeve 41 by the pumping magnetic pole S1 without going through the stirring and transporting process by the first screw 45a and the second screw 45b. If the developer with a reduced toner concentration is transported to the development region, an image with a reduced image concentration will be output. The phenomenon in which the developer is transferred from the peeling magnetic pole S3 to the pumping magnetic pole S1 due to insufficient developer peeling is called the drag phenomenon.

[0027] As shown in FIG. 2, in the first embodiment, when viewed in a cross section perpendicular to the rotation axis of the developing sleeve 41, the position where the regulating blade 43 is closest to the developing sleeve 41 is disposed vertically below the center of rotation of the developing sleeve 41. This type of system (lower blade system) has the following features compared to a system (upper blade system) in which the position where the regulating blade 43 is closest to the developing sleeve 41 is disposed vertically above the center of rotation of the developing sleeve 41. That is, compared to the upper blade system, the lower blade system can stabilize the amount of developer pumped up to the developing sleeve 41 even when the amount of developer present in the developing chamber 44a is small. On the other hand, in the lower blade system, gravity acts on the developer peeled off in the peeling area in the direction of the pumping magnetic pole S1. For this reason, the lower blade system is more likely to cause the drag phenomenon compared to the upper blade system.

[0028] In order to prevent the drag phenomenon from occurring, it is necessary to widen the gap between the peeling pole S3 and the pumping pole S1. When the outer diameter of the developing sleeve 41 is 18 mm, it is desirable to set the angle between the peeling pole S3 and the pumping pole S1 to 110° or more, thereby making the distance along the outer circumferential surface of the developing sleeve 41 between the peeling pole and the pumping pole 18 mm or more.

[0029] The distance along the outer circumferential surface of the developing sleeve 41 between the peeling pole S3 and the pumping pole S1 means the following. That is, it is the distance along the outer circumferential surface of the developing sleeve 41 between the position where the magnetic flux density Br of the normal direction component of the peeling pole reaches its maximum peak (maximum value) and the position where the magnetic flux density Br of the normal direction component of the pumping pole S1 reaches its maximum peak (maximum value). Moreover, the magnetic flux density Br of the normal direction component of the peeling pole S3 means the magnetic flux density of the peeling pole S3 in the normal direction of the developing sleeve 41. Moreover, the magnetic flux density Br of the normal direction component of the pumping pole S1 means the magnetic flux density of the pumping pole S1 in the normal direction of the developing sleeve 41.

[0030] In order to improve the developer stripping property, it is necessary to secure a sufficient distance along the outer circumferential surface of the developing sleeve 41 between the magnetic poles of the same polarity that form the stripping area (between the stripping magnetic pole S3 and the drawing magnetic pole S1). However, in recent years, the developing device has become smaller, and the outer diameter of the developing sleeve has become smaller accordingly (specifically, the outer diameter of the developing sleeve is 20 [mm] or less). If the number of magnetic poles of the magnet is the same, the smaller the outer diameter of the developing sleeve, the shorter the distance along the outer circumferential surface of the developing sleeve between the magnetic poles tends to be less than 18 [mm]. As a result, the distance along the outer circumferential surface of the developing sleeve between the magnetic poles of the same polarity that form the stripping area cannot be secured sufficiently, and the stripping area becomes narrow, so that the developer used for development becomes difficult to separate from the developing sleeve (the developer stripping property is reduced). This becomes a significant issue, particularly when the outer diameter of the developing sleeve is reduced in order to reduce the size of the developing device, and a lower blade system, which tends to cause the entrainment phenomenon more easily than an upper blade system, is adopted for the configuration of the developing device.

[0031] In the first embodiment, the outer diameter of the developing sleeve 41 is reduced (specifically, the outer diameter of the developing sleeve is set to 20 mm or less), and a lower blade system is adopted as a configuration of the developing device. Specifically, the outer diameter of the developing sleeve 41 is set to 18 mm, and the angle between the stripping pole S3 and the pumping pole S1 is set to 104.2°, so that the distance between the stripping pole S3 and the pumping pole S1 along the outer circumferential surface of the developing sleeve 41 is set to 16.4 mm. In the first embodiment, in a configuration in which the distance between the stripping pole S3 and the pumping pole S1 along the outer circumferential surface of the developing sleeve 41 is shorter than 18 mm, both the improvement of the developer stripping performance and the miniaturization of the device are achieved. Details of this will be described below.

[0032] (Characteristic Part of the Invention) Here, we will explain the force that attracts the developer to the developing sleeve 41. The force that attracts the developer to the developing sleeve 41 can be expressed by a magnetic attraction force Fr obtained by the following formula (Mathematical Expression 1).

[0033]

number

[0034] In the formula, μ is the magnetic permeability of the magnetic carrier, μ0 is the magnetic permeability of a vacuum, and b is the radius of the magnetic carrier. Also, Br is the component of the magnetic flux density in the normal direction of the developing sleeve 41. Br was measured using a magnetic field measuring device "MS-9902" (product name) manufactured by FWBELL as the measuring device, with the distance between the probe, which is a component of the measuring device, and the surface of the developing sleeve 41 being approximately 100 μm.

[0035] Bθ is calculated from the value of Br measured by the above method according to the following formula.

[0036]

number

[0037] Using the value of Br measured in this manner, Bθ can be derived and Fr can be calculated.

[0038] FIG. 4 shows a graph showing the distribution of magnetic flux density Br (hereinafter, simply referred to as Br) in the normal direction of the developing sleeve 41 in the configurations of "Example 1", "Example 2", and "Comparative Example" showing a conventional Br pattern example as an example of a Br pattern for obtaining the effect of the present invention. FIG. 4(b) is an enlarged view of the periphery of the peeling region. In FIG. 4, the N pole is the positive direction of Br, and the S pole is the negative direction. The difference between Example 1, Example 2, and Comparative Example is with respect to the distribution of Br in the periphery of the peeling region. On the other hand, the configurations with respect to the distribution of Br in other regions other than the peeling region (for example, the periphery of the development region) are assumed to be substantially the same in each of Example 1, Example 2, and Comparative Example.

[0039] As described above, in the first embodiment, the distance along the outer circumferential surface of the developing sleeve 41 between the peeling pole S3 and the pumping pole S1 is 16.4 mm, and the rotation angle of the developing sleeve 41 is 104.2°. In Fig. 4, Br when the fifth mag piece 42e (mag piece constituting the peeling pole S3) and the first mag piece 42a (mag piece constituting the pumping pole S1) are magnetized and positioned is shown as a comparative example.

[0040] In Example 1, the mag pieces 42a-e are magnetized and arranged in the same manner as in the comparative example, and then an additional magnetization process is added between the fifth mag piece 42e and the first mag piece 42a to intentionally change the Br shape around the peeled area. In Example 2, compared to Example 1, the strength of magnetization is increased when an additional magnetization process is added between the fifth mag piece 42e and the first mag piece 42a.

[0041] In the separation region, in Example 1, Br changes suddenly from the separation pole S3, followed by a region in which the absolute value of Br is small and the change in Br is small, and then the change in Br changes suddenly again toward the pumping pole S1, as compared to the comparative example. In addition to the characteristic of the change in Br in Example 1, Example 2 has the characteristic that the minimum peak value (minimum value) of Br in the separation region is large.

[0042] Here, the absolute value of the minimum peak value (minimum value) of Br in the peeling region (i.e., the region downstream of the maximum peak position (maximum peak position) of Br at peeling pole S3 and upstream of the maximum peak position (maximum peak position) of Br at pumping pole S1 in terms of the rotation direction of the developing sleeve 41) is defined as the "Br minimum value." In addition, as an index showing the state of change in Br in the peeling region, the width of the value obtained by adding 2 [mT] to the Br minimum value in the polarity direction of the magnetic poles forming the peeling region is defined as the "(Br minimum value + 2mT) width." The larger the "(minimum value + 2mT) width," the greater the degree of behavior in which Br changes rapidly from peeling pole S3, followed by a region in which the absolute value of Br is small and the change in Br is also small, and then the Br changes rapidly again toward pumping pole S1.

[0043] Table 1 shows characteristic configurations of the Br patterns in the configurations of Example 1, Example 2, and Comparative Example.

[0044] [Table 1]

[0045] As shown in Table 1, compared to the comparative example, Example 1 and Example 2 are characterized in that the distance and angle along the outer circumferential surface of the developing sleeve 41 between the peeling magnetic pole S3 and the pumping magnetic pole S1 are equal, but the "(Br minimum value + 2 mT) width" is larger. Also, compared to Example 1, Example 2 is characterized in that the absolute value (Br minimum value) of the minimum peak value (minimum value) of Br in the peeling region is larger.

[0046] Here, for the Br patterns of Example 1, Example 2, and Comparative Example shown in FIG. 4, the Fr patterns obtained by calculating Fr according to Equation 1 and Equation 2 are shown in FIG. 5. FIG. 5 is a diagram showing the distribution of Fr in the configurations of Example 1, Example 2, and Comparative Example. FIG. 5(b) is an enlarged view of the periphery of the peeled region. Note that Fr is calculated assuming that the relative permeability of the magnetic carrier is 5 and the radius of the magnetic carrier is 25 μm. Ta 5, the angle on the horizontal axis is defined as 0° in the vertically upward direction, and the rotation direction of the developing sleeve 41 is defined as the positive direction.

[0047] Of the forces acting on the magnetic carrier, Fr indicates a force in the normal direction of the developing sleeve 41 (magnetic attractive force in the normal direction of the developing sleeve 41). When Fr is a negative value, a force that attracts the magnetic carrier (developer) toward the center of the developing sleeve 41 acts on the magnetic carrier (developer), and when Fr is a positive value, a force (repulsive force) that causes the developer to be peeled off from the developing sleeve 41 acts on the developer. In other words, when the magnetic attractive force Fr in the normal direction of the developing sleeve 41 attracts the magnetic carrier (developer) toward the center of the developing sleeve 41 in a negative direction, the direction of the repulsive force is positive.

[0048] That is, developer peeling occurs in the region where Fr>0. The wider the region where Fr>0, the better the developer peeling performance, while the narrower the region where Fr>0, the more likely the drag phenomenon occurs. Hereinafter, the region where Fr>0 is called the "repulsive force width." Table 2 shows the distance of the repulsive force width along the outer circumferential surface of the developing sleeve 41 and the rotation angle of the developing sleeve 41 in each of the Br patterns of Example 1, Example 2, and the conventional example.

[0049] [Table 2]

[0050] From Table 2, it can be seen that the repulsive force width is larger in Example 1 and Example 2 than in the Comparative Example. Here, the reason why the repulsive force width is larger in the configuration of Example 1 than in the Comparative Example will be explained. In the formula 1, the part that contributes most to the increase in the repulsive force width is the "∂Br / ∂r" part. Figure 6 shows ∂Br / ∂r in the Br patterns in the configurations of Example 1 and the Comparative Example. Figure 6(b) is an enlarged view of the periphery of the peeled region.

[0051] As shown in Fig. 4(b), Br always has a negative value (south pole) in the peeled region. Therefore, when ∂Br / ∂r becomes negative in the peeled region, the first term of Equation 1 becomes a positive value (repulsive force). As shown in Fig. 6, the region where ∂Br / ∂r becomes negative is wider in Example 1 than in the conventional example.

[0052] This is because, by increasing the "(Br minimum value + 2 mT) width", the boundary between the region where the magnetic flux density Br becomes stronger the further away from the developing sleeve 41 and the region where the magnetic flux density Br becomes weaker the further away from the developing sleeve 41 has been moved further outward. This causes the region where the first term of equation 1 is a positive value to become wider, and therefore the repulsive force width where Fr>0 becomes wider.

[0053] Moreover, the repulsive force width of the configuration of the second embodiment is further increased compared to that of the first embodiment, because the value of the first term in Equation 1 is increased by increasing the value of the Br minimum value. Considering only this effect, the repulsive force width increases as the Br minimum value increases. However, if the Br minimum value is excessively large, the region in which ∂Br / ∂r takes a negative value becomes narrow, and as a result, there is a risk that the repulsive force width decreases. This decrease in the repulsive force width becomes significant when the Br minimum value is 9 [mT] or more. Therefore, in order to secure the repulsive force width, it is preferable that the Br minimum value in the peeled region is 0 [mT] or more and 9 [mT] or less. In order to obtain the effect of further increasing the repulsive force width as shown in the second embodiment, it is more preferable that the Br minimum value in the peeled region is 3 [mT] or more and 9 [mT] or less.

[0054] In order to confirm the effect of the first embodiment, the decrease in image density was measured when 10 sheets of A4 solid images were printed continuously using the configurations of Example 1, Example 2, and Comparative Example. The density value was measured using a reflection spectrodensitometer 500 series manufactured by X-Rite Corporation, and the average value of the values ​​of 10 points in the main scanning direction at the center of the output image in the sub-scanning direction was used.

[0055] When the difference in reflection density between the first sheet and the tenth sheet is taken as Δ, Δ=0.05 in the configuration of Example 1, Δ=0.02 in the configuration of Example 2, and Δ=0.22 in the configuration of the comparative example. In other words, the results show that the drag phenomenon is suppressed in Examples 1 and 2 compared to the comparative example.

[0056] In the first embodiment described above, the distance between the peeling pole S3 and the pumping pole S1 along the outer circumferential surface of the developing sleeve 41 is shorter than 18 mm, specifically, the distance is 16.4 mm, which corresponds to a rotation angle of 104.2° of the developing sleeve 41. In the comparative example, when the distance between the peeling pole S3 and the pumping pole S1 along the outer circumferential surface of the developing sleeve 41 is shorter than 18 mm, the normal Br pattern causes the drag phenomenon. On the other hand, in the first and second embodiments, even when the distance between the peeling pole S3 and the pumping pole S1 along the outer circumferential surface of the developing sleeve 41 is short, the repulsive force width is widened by devising the Br pattern in the peeling region, the peeling property is improved, and the drag phenomenon is suppressed.

[0057] The distance between the peeling magnetic pole S3 and the drawing magnetic pole S1 along the outer circumferential surface of the developing sleeve 41 was fixed at 16.4 mm, the minimum Br value was fixed at 3.0 mT, and the "minimum Br value + 2 mT" width was further varied to conduct a similar study. The repulsive force width, the reflection density difference Δ between the first and tenth sheets, and the density difference between the first and tenth sheets were visually confirmed, and the results are shown in Table 3.

[0058] In addition, the visual evaluation is as follows: differences that are clearly unacceptable are marked with an ×; differences that are within the acceptable limit are marked with a △; slight differences that are within the acceptable limits are marked with an ◯; and differences that cannot be confirmed visually are marked with an ◎.

[0059] [Table 3]

[0060] In order to suppress the drag phenomenon to an acceptable level as a decrease in image density, the repulsive force width where the magnetic attractive force Fr in the normal direction of the developing sleeve 41 is a positive value needs to be 4.7 [mm] as the distance along the outer circumferential surface of the developing sleeve 41 and 30° or more as the rotation angle. In order to further enhance the effect of suppressing the drag phenomenon, it is more preferable to set the distance along the outer circumferential surface of the developing sleeve 41 to 5.7 [mm] and the rotation angle to 36° or more. To achieve this, the "(Br minimum value + 2 mT) width" needs to be 5.5 [mm] as the distance on the outer circumferential surface of the developing sleeve 41 and 35° or more as the rotation angle. It is preferable to set the distance along the outer circumferential surface of the developing sleeve 41 to 6.3 [mm] and the rotation angle to 40° or more.

[0061] As a means for widening the "(Br minimum value + 2 mT) width", in addition to devising the magnetization conditions when magnetizing the mag pieces 42a-e as in the first embodiment, it is possible to devise the shape of the mag pieces 42a-e in the magnet 42. Whichever means is used, it is possible to obtain the effect of the first embodiment.

[0062] As described above, in the first embodiment, the distance between the peeling magnetic pole S3 and the pumping magnetic pole S1 along the outer circumferential surface of the developing sleeve 41 is shorter than 18 mm. In the first embodiment, the absolute value of the minimum peak value (minimum value) of Br in the peeling region is defined as the "Br minimum value", and the width of the value obtained by adding 2 mT to the Br minimum value is defined as the "(Br minimum value + 2 mT) width" as an index showing the state of change in the peeling region. In the first embodiment, in a configuration in which the distance between the magnetic poles of the same polarity (between the peeling magnetic pole and the pumping magnetic pole) that form the peeling region along the outer circumferential surface of the developing sleeve is short, the "(Br minimum value + 2 mT) width" is widened as the distribution of Fr in the peeling region. In such a first embodiment, the developer peeling property can be guaranteed even if the developing device 44 is made smaller and the outer diameter of the developing sleeve 41 is made smaller, so that both the improvement of the developer peeling property and the miniaturization of the device can be achieved.

[0063] (Other embodiments) The present invention is not limited to the above-described embodiments, and various modifications (including organic combinations of the respective embodiments) are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0064] In the above embodiment, as shown in FIG. 2, the magnet 42 has a plurality of magnetic poles, and the number of magnetic poles is five. However, the present invention is not limited to this. In order to separate the developer by the repulsive magnetic field between the magnetic poles of the same polarity, the number of magnetic poles of the magnet 42 may be an odd number. Specifically, the present invention can be applied to the number of magnetic poles of the magnet 42 being three or seven. If the outer diameter of the developing sleeve 41 is the same, the greater the number of magnetic poles of the magnet 42, the shorter the distance along the outer circumferential surface of the developing sleeve 41 between the separating magnetic pole and the pumping magnetic pole tends to be. That is, if the outer diameter of the developing sleeve 41 is the same, when the number of magnetic poles of the magnet 42 is five, the entrainment phenomenon is more likely to occur than when the number of magnetic poles of the magnet 42 is three. If the outer diameter of the developing sleeve 41 is the same, when the number of magnetic poles of the magnet 42 is seven, the entrainment phenomenon is more likely to occur than when the number of magnetic poles of the magnet 42 is five.

[0065] In the above embodiment, as shown in Fig. 1, an image forming apparatus using an intermediate transfer belt 62 has been described as an example, but the present invention is not limited to this. It is also possible to apply the present invention to an image forming apparatus configured to transfer a recording material by directly contacting the recording material with the photosensitive drums 1 (1A, 1B, 1C, 1D) in order.

[0066] In the above embodiment, the developing device 4 in which the developing sleeve 41 rotates counterclockwise (in the direction of the arrow in FIG. 2) and a lower blade system is used has been described as an example, but the present invention is not limited to this. It is also possible to apply the present invention to a developing device 4 in which the developing sleeve 41 rotates clockwise and an upper blade system is used.

[0067] In the above embodiment, the developing device 4 is configured such that the developing chamber 44a and the stirring chamber 44b are arranged side by side in the horizontal direction as shown in Fig. 3, but the present invention is not limited to this. It is also possible to apply the present invention to a developing device 4 configured such that the developing chamber 44a and the stirring chamber 44b are arranged side by side vertically in the direction of gravity. [Explanation of symbols]

[0068] 4. Developing device 41 Developing sleeve 42 Magnet

Claims

1. a rotatable developing sleeve that carries and conveys a developer containing toner and carrier for developing an electrostatic latent image formed on an image carrier; a magnet that is fixedly disposed inside the developing sleeve in a non-rotatable manner, the magnet having a first magnetic pole and a second magnetic pole that is disposed downstream of the first magnetic pole in the rotational direction of the developing sleeve and adjacent to the first magnetic pole and has the same polarity as the first magnetic pole; Equipped with a distance along an outer circumferential surface of the developing sleeve from a first maximum peak position where the magnetic flux density of the first magnetic pole is a maximum value in the normal direction of the developing sleeve to a second maximum peak position where the magnetic flux density of the second magnetic pole is a maximum value in the normal direction of the developing sleeve is shorter than 18 mm; When the direction of the magnetic attraction force in the normal direction of the developing sleeve that attracts the developer toward the center of the developing sleeve is defined as negative, the distance along the outer circumferential surface of the developing sleeve of the repulsive force width at which the magnetic attraction force in the normal direction of the developing sleeve is a positive value downstream of the first maximum peak position and upstream of the second maximum peak position in the rotational direction of the developing sleeve is 4.7 mm or more. A developing device characterized by the above.

2. Downstream of the first maximum peak position and upstream of the second maximum peak position in the rotational direction of the developing sleeve, the distance along the outer circumferential surface of the developing sleeve of the width of the value obtained by adding 2 [mT] to the value at which the magnetic flux density in the normal direction of the developing sleeve is minimum is 5.5 [mm] or more.

2. The developing device according to claim 1,

3. When the direction of the magnetic attraction force with respect to the normal direction of the developing sleeve that attracts the developer toward the center of the developing sleeve is negative, the distance of the repulsive force width along the outer peripheral surface of the developing sleeve downstream of the first maximum peak position and upstream of the second maximum peak position with respect to the rotational direction of the developing sleeve is 5.7 mm or more.

2. The developing device according to claim 1,

4. The distance along the outer circumferential surface of the developing sleeve of the width of the value obtained by adding 2 [mT] to the value at which the magnetic flux density in the normal direction of the developing sleeve is minimum downstream of the first maximum peak position and upstream of the second maximum peak position in relation to the rotational direction of the developing sleeve is 6.3 [mm] or more.

4. The developing device according to claim 3.

5. The value at which the magnetic flux density in the normal direction of the developing sleeve is minimum downstream of the first maximum peak position and upstream of the second maximum peak position in the rotation direction of the developing sleeve is 0 [mT] or more and 9 [mT] or less.

5. The developing device according to claim 1, wherein the developing device is a developing unit.

6. A value at which the magnetic flux density in the normal direction of the developing sleeve is minimum downstream of the first maximum peak position and upstream of the second maximum peak position in the rotation direction of the developing sleeve is 3 mT or more and 9 mT or less.

5. The developing device according to claim 1, wherein the developing device is a developing unit.

7. The outer diameter of the developing sleeve is 20 mm or less.

7. The developing device according to claim 1, wherein the developing device is a developing unit.

8. The magnet has a plurality of magnetic poles, The number of the plurality of magnetic poles is five.

8. The developing device according to claim 1, wherein the developing device is a developing unit.

9. The magnet has a plurality of magnetic poles, The number of the plurality of magnetic poles is seven.

8. The developing device according to claim 1, wherein the developing device is a developing unit.

10. a regulating member for regulating an amount of developer carried on the developing sleeve, When viewed in a cross section perpendicular to the rotation axis of the developing sleeve, the position where the regulating member is closest to the developing sleeve is vertically below the rotation center of the developing sleeve.

10. The developing device according to claim 1, wherein the developing device is a developing unit.

11. Further comprising a regulating member for regulating the amount of developer carried on the developing sleeve, the magnet further has a third magnetic pole disposed upstream of the first magnetic pole and downstream of the second magnetic pole in a rotation direction of the developing sleeve, The regulating member regulates the amount of developer carried on the developing sleeve in a state in which a magnetic brush is formed on the outer circumferential surface of the developing sleeve by the third magnetic pole.

10. The developing device according to claim 1, wherein the developing device is a developing unit.

12. When viewed in a cross section perpendicular to the rotation axis of the developing sleeve, the position where the regulating member is closest to the developing sleeve is vertically below the center of rotation of the developing sleeve. The developing device according to claim 11 .

13. a plurality of grooves are formed on an outer circumferential surface of the developing sleeve along a rotation axis of the developing sleeve; the plurality of grooves are formed over an entire outer circumferential surface of the developing sleeve in a rotational direction of the developing sleeve, The grooves are formed at predetermined intervals in the rotation direction of the developing sleeve.

13. The developing device according to claim 1, wherein the developing device is a developing unit.

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